As with members of all categories of semi-synthetic opioids derived from morphine (and codeine is one of these, in fact) with the exception of the simple esters of morphine like heroin, nicomorphine, acetyldipropanoylmorphine &c, hydrocodone made commercially is manufactured in a much more efficient synthesis from thebaine, a close chemical relative of codeine found in fairly high concentrations in opium. Hydrocodone is two steps from codeine and therefore three from morphine, and in fact a number of syntheses were found, with others being from dihydrocodeine, dihydromorphine, tetrahydrothebaine and a large number of total syntheses from coal tar or petroleum through morphine, codeine, and thebaine. Hydrocodone's closest structural relatives are fellow 14-hydroxycodeineones such as oxycodone, oxymorphone, hydromorphinol, oxymorphol, and hydromorphone. Much of hydrocodone's activity is from the last of these produced in the liver from the hydrocodone, in the one step which activates similar drugs by turning them into stronger derivatives in the same fashion in which the liver turns codeine to morphine. However, hydrocodone is also turned into other active products by other metabolic pathways and hydrocodone does, like oxycodone, have intrinsic activity of its own to a greater extent than is the case for codeine.

Hydrocodone is prescribed predominantly within the United States, with the International Narcotics Control Board reporting that 99% of the worldwide supply in 2007 was consumed in the United States. Several common imprints for hydrocodone are M365, M366, M367.[12]

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Hydrocodone is used to treat moderate to severe pain, although it is often prescribed to treat mild pain as well.
In liquid formulations, it is used as an antitussive to treat cough.[9] In one study comparing the potency of hydrocodone to that of oxycodone, it was found that it took 50% more hydrocodone to achieve the same degree of miosis (pupillary contraction).[13] The investigators interpreted this to mean that oxycodone is about 50% more potent than hydrocodone.

However, in a study of emergency department patients with fractures, it was found that an equal amount of either drug provided about the same degree of pain relief, indicating that there is little practical difference between them when used for that purpose.[14] Some references state that the analgesic action of hydrocodone begins in 20–30 minutes and lasts about 4–8 hours.[15] The manufacturer's information says onset of action is about 10–30 minutes and duration is about 4–6 hours.[16] Recommended dosing interval is 4–6 hours.

Hydrocodone is available in a variety of formulations for oral administration:[17][18][19]

The original oral form of hydrocodone alone, Dicodid, as immediate-release 5 and 10 mg tablets of the hydrochloride (by analogy to Dilaudid, the original hydromorphone product invented around the same time, both in Germany) is available for prescription in Continental Europe per national drug control and prescription laws and Title 76 of the Schengen Treaty, but dihydrocodeine has been much more widely used for the same indications since the beginning in the early 1920s, with hydrocodone being regulated the same way as morphine in the German Betäubungsmittelgesetz, the similarly named law in Switzerland and the Austrian Suchtmittelgesetz, whereas dihydrocodeine is regulated like codeine. For a number of decades, the liquid hydrocodone products available are cough medicines.

Hydrocodone plus homatropine (Hycodan) in the form of small tablets for coughing and especially neuropathic moderate pain (the homatropine, an anticholinergic, is useful in both of those cases and is a deterrent to intentional overdose) was more widely used than Dicodid and was labelled as a cough medicine in the United States whilst Vicodin and similar drugs were the choices for analgesia.

Extended-release hydrocodone in a time-release syrup also containing chlorphenamine/chlorpheniramine is a cough medicine called Tussionex in North America. In Europe, similar time-release syrups containing codeine (numerous), dihydrocodeine (Paracodin Retard Hustensaft), nicocodeine (Tusscodin), thebacon, acetyldihydrocodeine, dionine, and nicodicodeine are used instead.

Several cases of progressive bilateral hearing loss unresponsive to steroid therapy have been described as an infrequent adverse reaction to hydrocodone/paracetamol misuse. This adverse effect has been considered by some to be due to the ototoxicity of hydrocodone.[22][23] Other researchers have suggested that paracetamol is the primary agent responsible for the ototoxicity.[24][25]

Hydrocodone is in U.S. Food and Drug Administration (FDA) pregnancy category C. No adequate and well-controlled studies in humans have been conducted. A newborn of a mother taking opioid medications regularly prior to the birth will be physically dependent. The baby may also exhibit respiratory depression if the opioid dose was high.[26] An epidemiological study indicated that opioid treatment during early pregnancy results in increased risk of various birth defects.[27]

Studies have shown hydrocodone is stronger than codeine but only one-tenth as potent as morphine at binding to receptors and reported to be only 59% as potent as morphine in analgesic properties. However, in tests conducted on rhesus monkeys, the analgesic potency of hydrocodone was actually higher than morphine.[5]Oral hydrocodone has a mean equivalent daily dosage (MEDD) factor of 0.4, meaning that 1 mg of hydrocodone is equivalent to 0.4 mg of intravenous morphine. However, because of morphine's low oral bioavailability, there is a 1:1 correspondence between orally administered morphine and orally administered hydrocodone.[35] The relative milligram strength of hydrocodone to codeine is given as 6 fold, that is 5 mg has the effect of 30 mg of codeine; by way of the Roman numeral VI this is said to have given rise to the trade name Vicodin.[citation needed]

Hydrocodone is only pharmaceutically available as an oral medication.[1] It is well-absorbed, but its oral bioavailability of hydrocodone is only approximately 25%.[3][4] The onset of action of hydrocodone via this route is 10 to 20 minutes, with a peak effect (Tmax) occurring at 30 to 60 minutes,[36] and it has a duration of 4 to 8 hours.[1]

In the liver, hydrocodone is transformed into several metabolites, including norhydrocodone, hydromorphone, 6α-hydrocodol (dihydrocodeine), and 6β-hydrocodol.[2] 6α- and 6β-hydromorphol are also formed, and the metabolites of hydrocodone are conjugated (via glucuronidation).[37][38] Hydrocodone has a terminal half-life that averages 3.8 hours (range 3.3–4.4 hours).[5][1] The hepatic cytochrome P450 enzyme CYP2D6 converts hydrocodone into hydromorphone, a more potent opioid (5-fold higher binding affinity to the MOR).[2][39] However, extensive and poor cytochrome 450 CYP2D6 metabolizers had similar physiological and subjective responses to hydrocodone, and CYP2D6 inhibitor quinidine did not change the responses of extensive metabolizers, suggesting that inhibition of CYP2D6 metabolism of hydrocodone has no practical importance.[40][41] Ultra-rapid CYP2D6 metabolizers (1–2% of the population) may have an increased response to hydrocodone; however, hydrocodone metabolism in this population has not been studied.[42]

Norhydrocodone, the major metabolite of hydrocodone, is predominantly formed by CYP3A4-catalyzed oxidation.[2] In contrast to hydromorphone, it is described as inactive.[39] However, norhydrocodone is actually a MOR agonist with similar potency to hydrocodone, but has been found to produce only minimal analgesia when administered peripherally to animals (likely due to poor blood–brain barrier and thus central nervous system penetration).[43] Inhibition of CYP3A4 in a child who was, in addition, a poor CYP2D6 metabolizer, resulted in a fatal overdose of hydrocodone.[44] Approximately 40% of hydrocodone metabolism is attributed to non-cytochrome P450-catalyzed reactions.[45]

Hydrocodone concentrations are measured in blood, plasma, and urine to seek evidence of misuse, to confirm diagnoses of poisoning, and to assist in investigations into deaths. Many commercial opiate screening tests react indiscriminately with hydrocodone, other opiates, and their metabolites, but chromatographic techniques can easily distinguish hydrocodone uniquely. Blood and plasma hydrocodone concentrations typically fall into the 5–30 µg/L range among people taking the drug therapeutically, 100–200 µg/L among recreational users, and 100–1,600 µg/L in cases of acute, fatal overdosage. Co-administration of the drug with food or alcohol can very significantly increase the resulting plasma hydrocodone concentrations that are subsequently achieved.[46][47]

Hydrocodone was first marketed by Knoll as Dicodid, starting in February 1924 in Germany. This name is analogous to other products the company introduced or otherwise marketed: Dilaudid (hydromorphone, 1926), Dinarkon (oxycodone, 1917), Dihydrin (dihydrocodeine, 1911), and Dimorphan (dihydromorphine). Paramorfan is the trade name of dihydromorphine from another manufacturer, as is Paracodin, for dihydrocodeine.[citation needed]

The name Dicodid was registered in the United States and appears without a monograph as late as 1978 in the Physicians' Desk Reference; Dicodid may have been marketed to one extent or another in North America in the 1920s and early 1930s. The drug was pure hydrocodone in small 5 and 10 mg tablets, physically similar to the Dilaudid tablets. It is no longer manufactured by Knoll in Germany, nor is a generic available. Hydrocodone was never as common in Europe as it is in North America—dihydrocodeine is used for its spectrum of indications. Germany was the number two consumer of hydrocodone until the manufacture of the drug was discontinued there. Now,[when?] the world outside the United States accounts for less than 1% of annual consumption. It was listed as a Suchtgift under the German Betäubungsmittelgesetz and regulated like morphine. It became available in the Schengen Area of the European Union as of 1 January 2002 under Title 76 of the Schengen Treaty.[citation needed]

Most hydrocodone is formulated in combination with a second analgesic, such as paracetamol (acetaminophen) or ibuprofen. Examples of hydrocodone combinations include Norco, Vicodin, Lortab, Vicoprofen and Riboxen.[51]

In 2014, the FDA approved a formulation of hydrocodone called Zohydro ER made by Zogenix Pharmaceuticals. The approval of Zohydro ER was controversial, due to concerns over its potential for substance abuse. The FDA approved Zohydro ER over the objections of its own review panel, which voted 12 to 2 against approval. The panel stated that if approved, Zohydro ER would likely "be abused, possibly at a rate greater than that of currently available hydrocodone combination products". Thirty U.S. states asked the FDA not to approve Zohydro ER in capsule form due to its potency and the ease with which it could be abused, by being crushed and then snorted or injected.[53]

The US government imposed tougher prescribing rules for hydrocodone in 2014, changing the drug from Schedule III to Schedule II.[56][57][58][59] In 2011, hydrocodone products were involved in around 100,000 abuse-related emergency department visits in the United States, more than double the number in 2004.[60]